SEOUL (AFP) - North Korea put its military on combat alert Monday as US and South Korean troops began a major joint exercise, and warned that any attempt to block its upcoming satellite launch would spark a war. The communist state also severed its last communications channel with South Korea for the duration of the 12-day exercise, which Pyongyang has branded a rehearsal for invasion. The North Korean military described the exercise as "unprecedented in the number of the aggressor forces involved and in duration." "The KPA (Korean People's Army) Supreme Command issued an order to all service persons to be fully combat-ready," it said in a statement carried by official media. "A war will break out if the US imperialists and the warmongers of the South Korean puppet military hurl the huge troops and sophisticated strike means to mount an attack." The North said it was switching off military phone and fax lines, which are used to approve border crossings, because it would be "nonsensical" to maintain normal channels during the drill. Some 80 South Koreans due to return Monday from the Kaesong joint industrial estate - built by Seoul in the North as a symbol of reconciliation - were stranded there. More than 700 others cancelled trips the other way, the unification ministry said. It expressed regret at the communications cut-off and urged Pyongyang to backtrack and stop raising tensions. This year's exercise involves an aircraft carrier, 26,000 US troops and more than 30,000 South Koreans. Seoul and Washington say it is purely defensive. It comes at a time of high cross-border tension and growing pressure on the North to drop plans to fire a rocket. North Korea says it is preparing to launch a satellite, but both Seoul and Washington believe the real purpose is to test a long-range Taepodong-2 missile that could in theory reach Alaska. The North's military General Staff warned it would retaliate "with prompt counter-strikes by the most powerful military means" for any attempt to intercept it. Analysts suspect the North is taking a tougher stance as it competes for US President Barack Obama's attention with other world hotspots. It is also angry at South Korea's conservative President Lee Myung-Bak, who has scrapped his predecessors' policy of offering virtually unconditional aid to Pyongyang. "Whether they describe it as a satellite launch or something else makes no difference, they would be in violation of UN Security Council Resolution 1718," the new US envoy for North Korea, Stephen Bosworth, said Monday. He has been holding talks in Beijing, Tokyo and Seoul on ways to persuade the North to push ahead with a six-nation nuclear disarmament deal and to hold back from firing a rocket. Bosworth said the five parties negotiating with Pyongyang strongly agree it would be "extremely ill-advised" for it to fire a rocket. The nuclear talks, involving the two Koreas, the United States, Japan, Russia and China, are deadlocked by disputes over how the North's declared atomic activities should be verified. "We are hopeful that we can see the resumption of the six-party process in the relatively near future," Bosworth told reporters after a day of talks with senior South Korean officials.